The present invention relates generally to electronic component assemblies that generate electromagnetic interference and in particular to shielding a component of an electronic component assembly from the electromagnetic interference generated by the assembly.
Electromagnetic interference (EMI) poses a persistent problem for electronic component assemblies. An active component in an electronic component assembly, typically a semiconductor device such as a radio frequency (RF) power transistor in an RF power amplifier or a regulating switching device in a switching power supply, may radiate spectral energy, that is, harmonics, when respectively amplifying an RF signal or regulating an output voltage. In addition, an RF signal propagating along a passive component such as a transmission line disposed on a substrate may radiate spectral energy due to a difference between a dielectric constant of the air above the transmission line and a dielectric constant of the substrate below the transmission line. The spectral energy radiates into the atmosphere in a form of electromagnetic waves. The radiated electromagnetic waves may then couple into other components of the electronic component assembly, creating undesirable EMI for signals propagating through the other components. Furthermore, when the electronic component assembly is part of a telecommunication system, EMI that escapes the assembly, such as EMI that mixes with an amplified RF signal prior to the signal being output by an RF power amplifier, can pose unacceptable interference for other users of the RF spectrum. The EMI coupling into an output signal of an RF power amplifier can also create power control problems by distorting RF feedback power.
In order to control an amount of EMI radiated or propagated by an electronic component assembly, the assembly typically includes a housing, such as a machined aluminum housing, that functions as an electromagnetic shield. The housing is designed to contain the EMI generated by the components of the assembly. However, the housing does not protect the components of the assembly from EMI generated by other components of the assembly. In order to protect individual components, such as an output transmission line, of the assembly from EMI, electronic component assemblies typically surround the component with a mechanical shield that blocks the EMI from reaching the component. However, use of a mechanical shield is expensive, poses manufacturing problems involved in a placement of, and subsequent soldering of, the shield on a substrate, and poses a potential heat dissipation problem when the shield encloses an area where a high power RF signal is propagating along an output transmission line and/or through components serially coupled to the output transmission line. Accordingly, EMI reduction poses a persistent design problem for both mechanical and electrical designers of high power electronic component assemblies.
Therefore, there is a need for shielding a component, or an output signal, of an electronic component assembly from EMI generated by other components of the assembly without using a mechanical shield.